This invention relates to an apparatus and method for measuring the concentration of solid particles in a fluid stream. More particularly, the invention relates to an apparatus and method in which measurement of the concentration of solid particles is effected by triboelectrically charging the particles in part of the stream and then measuring the charges upon the particles.
It is known to measure the concentration of solid particles in a gas by drawing a sample of the gas over an electrode. The impacts of particles in the gas sample upon the electrode cause frictional or triboelectric charging of the particles and the electrode and the resultant charge upon the electrode can be detected by appropriate circuitry, thereby giving a measurement of the concentration of particles in the gas. In one known form of such a device intended for use in sampling air within a building or plant, the electrode projects into the center of a venturi tube. Downstream from the electrode, motive air inlet tubes intersect the venturi. Air under pressure is fed into the motive air inlet tubes, thereby drawing air through the venturi tube over the electrode and permitting the electrode to measure the concentration of dust particles in the air sample.
It appears that this type of dust measuring device can give acceptable results when the air to be sampled is essentially static, as for example in a room. However, many of the most important applications of dust detecting devices involve determining the concentration of solid particles in moving gas streams. For example, dust detection devices may be used for checking the efficiency of dust removal equipment used in certain environmental control systems (e.g. removal of fibers from the air in textile mills or removal of dust in grain elevators) and for these purposes it will be desirable to install the dust detection apparatus in the moving stream of air immediately downstream of the dust removal equipment. Similarly, particle detection equipment may be used for detecting dust, soot, solid pollutants and other particles in waste gases, such as gases in flues, automobile exhaust systems, cement kilns and power generation plants, where streams of gas may be moving with considerable velocities. In many of these cases, the velocity of the gas stream can be expected to vary with time depending upon the level at which the plant is being operated.
One particularly important application of particle detection devices where wide variations in the velocity of the gas streams involved must be expected is monitoring of the efficiency of air cleaning systems used in conjunction with internal combustion engines and similar devices. Internal combustion engines require a large supply of clean air to ensure maximum engine performance and engine life and to reduce maintenance requirements. Air cleaning systems have been developed which will remove 99% of the particulate matter which is drawn into the air intake systems. Such high efficiency air cleaning systems are multi-stage units which include barrier type air filters. However, a simple dust leak in the air cleaning system (caused by, for example, accidental perforation of one of the air filters) can negate the effectiveness of the system. The vehicle operator must know when such a leak occurs so that he can shut off the engine and take steps to correct the leak problem before irreparable damage is done. This problem is especially of concern on military tanks equipped with gas turbine engines which are highly susceptible to damage by dust in the air under some operating conditions. Also, problems with excessively dusty air may be encountered in other internal combustion engines equipped with turbo chargers. A reliable dust detection system can also be valuable on any engine and on many other systems where filters are used to remove dust from the air to provide a supply of clean air for an air-consuming device. One type of particle detection device which relies upon triboelectric charging is disclosed in U.S. patent application Ser. No. 490,533 filed May 2, 1983 by myself and Loren R. Albrechtson, now U.S. Pat. No. 4,531,402, issued July 30, 1985.
One potential advantage of particle detectors relying upon triboelectric charging for monitoring the efficiency of air filtration systems is that such triboelectric particle detectors do have some capacity for distinguishing between large and small particles in a gas stream whereas, for example, filter-type particle detectors tend to measure only the total weight of the particles per unit volume without regard to their size. The ability of triboelectric particle detectors to distinguish between large and small particles is important because many engines and other gas-consuming devices can tolerate the presence of very small particles (such as those found in many smokes) but are highly susceptible to damage by relatively large particles (typically those exceeding 5 microns in diameter). If one fits an engine or similar gas-consuming device with a particle detector for measuring the concentration of particles in the air entering the engine or other device and this particle detector does not distinguish between large and small particles, the particle detector will yield a considerable number of "false alarms" when the incoming air contains small particles which are sufficient to actuate the particle detector but which would not actually be harmful to the engine or the device. Such false alarms are obviously undesirable since they unnecessarily increase the down time of the engine or other device and may result in unnecessary maintenance costs. This problem of false alarms is exacerbated by the fact that the air filtration system of an engine or other device which is not harmed by very small particles will often, in order to present as little resistance to air flow as possible, be designed so that such very small particles pass the air filtration system and only the larger, potentially damaging particles are retained by the system. Thus, triboelectric particle detectors are potentially valuable in monitoring the performance of many air filtration systems used in practice. In addition, such triboelectric particle detectors can be made relatively simple in construction; for example, in the type of static air particle detector described above which uses motive air inlet tubes to draw a sample of air through a venturi tube, it would appear that where this type of particle detector is to be used to sample a stream of air which is already moving, the motive air inlets could be omitted, thereby producing an extremely simple type of particle detector.
Unfortunately, I have found that it is difficult to obtain consistent readings from triboelectric particle detectors which are used to measure the concentration of particles in moving streams of air which are susceptible to considerable changes in velocity, such as those experienced by the streams passing through air filtration systems of internal combustion engines (where the air stream velocity varies substantially linearly with engine speed); as already noted, particle detectors are also used in several other fields where the velocity of the gas stream being monitored can vary over a wide range. I have found that, even though the concentration of particles in the gas stream passing through a triboelectric particle detector remains constant, the apparent particle concentration indicated by the triboelectric particle detector increases with increased gas velocity. Thus, when a triboelectric particle detector is being used to monitor air being supplied from an air filtration system to an internal combustion engine, at high engine speed the triboelectric particle detector may indicate a dangerously high particle concentration, although in fact the particle concentration is within the acceptable range. Such susceptibility to false alarms at high engine speeds is particularly unfortunate since the operator is most likely to be given a false alarm when the vehicle he is driving is travelling at high speed, perhaps upon an expressway or similar highway, when it may be most inconvenient to stop the vehicle in order to check the functioning of the air filtration system.
Accordingly, there is a need for a triboelectric particle detector capable of measuring particle concentration in moving streams of fluid which is not greatly affected by variations in the velocity of the fluid stream, and this invention seeks to provide such a particle detector and a method for its use.